Intent is a big part of the Oath.
The intent to deceive is what determines whether a word spoken is "not true". If the speaker believes it to be true, it is considered "true" for the purposes of the Oath. If Moiraine believes that she would destroy Rand before letting the Shadow take him, then her uttering that phrase would be true.
However... circumstances change, and words that were true can cease to be true. This does not seem to conflict with the Oath at all.
If an Aes Sedai said "I'm going to put this egg in the basket in the other room", and then went to find that the basket was no longer there, she has not violated her Oath, nor is she now compelled to Quest to find the basket, put it back in the room, and put that egg in it.
It is entirely possible to say you'll do something under the Oath, and then later change your mind and not do it. There's no way the Oath could operate without allowing for that, otherwise the attrition of Aes Sedai who said they were going to do something that they found they could not, or should not, follow through on would be so great that either they would have modified the Oath, or ceased to exist as an organization.
Formal oaths, however, have a stronger weight, and would be amplified by the Oath sworn on the Rod. Someone swearing "by the Light" or other variations of formal oaths would likely be bound to adhere to the terms of those oaths as best they could, if they were an Aes Sedai who had previously been bound by the Three Oaths.